The goal of the work proposed here is to develop a method for quantitating the simultaneous contributions of carbohydrate, lipid and amino acids to oxidative metabolism in the slow oxidative fibers of rat soleus muscle, and then to systematically evaluate the effects of changing substrate levels and finally the interaction of insulin with the direct substrate effects. Measurements will use a system for continuous monitoring of 14CO2 production and release of labelled compounds into the media. The analysis will be based on a steady state model of labelled substrate use which is most sensitive to metabolic branch points. Therefore both endogenous and exogenous substrate turnover can be quantitated.